There are many examples of SGML, many of them are more than twice that
old, and some are still in daily use. Do they count? I think the
question is important from the perspective of eternity. Technically
speaking, XML documents are SGML documents (it says here in ISO 8879 as
amended) but the reverse is generally not true. Will eternity ignore
the SGML because it's not XML? I doubt it.

Eternal? snort.

The text is not eternal, sorry, even if the ideas might in some sense
be, and the data format is definitely more short-lived than the text.
My reference point is OED: it started as little slips of paper, became
(somehow) a bunch of books. That form lasted quite long (80+ years?).
Then there was OED2, another book, and the first digital edition: it was
delivered to the web platform as SGML, and rendered as HTML, which was
how most people saw it. That lasted about 10 years. OED3 (which I
worked on) was passed to us as XML, and although we also published OED2
entries as a secondary resource, I never saw the SGML - it is preserved
now primarily as HTML, because there's no need to treat it as a living
text any more. That's almost 4 years old. We'll see how long it lasts,
but I doubt it'll make it to 10 before some other form takes it place.